Tens of thousands of people Sunday showed their opposition to the Keystone XL pipeline as they marched in cold weather to the White House to urge President Obama to reject the $5.3-billion project, which critics say would hurt the environment.

Organizers estimated that roughly 35,000 people from several U.S. states gathered on the National Mall and then marched to Constitution Avenue, up 17th Street and past the White House, and billed it as the largest climate rally in American history.

"All I ever wanted to see was a movement of people to stop climate change and now I've seen it," Politico quoted Bill McKibben, founder of 350.org, as telling the crowd of protesters. "I cannot promise you we're going to win, but I've waited a quarter century to find out if we were gonna fight. And today, at the biggest climate rally by far, by far, by far, in U.S. history – today, I know we're going to fight."

McKibben's group was one of the three organizers, apart from the Sierra Club and the Hip Hop Caucus. Sunday's rally was held following a call from a bipartisan group of senators to approve the project.

Critics say the TransCanada Corp.'s pipeline project will hurt the environment by encouraging development of Alberta's oil sands, the mining of which releases three times more carbon dioxide, believed to be a greenhouse gas, than conventional crude oil. But its supporters claim the pipeline will lead to job creation and less foreign dependence on oil.

"It will be a punch in the gut to all the activists who worked so hard to get the president elected" if Keystone is approved, Michael Brune, the executive director of the Sierra Club, was quoted as saying.

"I don't have any reason to believe at this point that the pipeline won't go through," said Van Jones, founder of Rebuild the Dream and former Obama special adviser for green jobs. "I think if you look at the status quo, the big polluters tend to win these fights. They've got the big money. They've got the big lobbyists and they tend to win these fights. And that's why the people have to come out."

Obama was reportedly playing golf in Florida when the rally was being held.

TransCanada promises that its 2,154-mile international tar sands pipeline extending from Alberta, Canada, to U.S. markets in Illinois, Nebraska and Oklahoma will create "more than $20 billion in new private sector investment in the U.S. economy" as well as create 20,000 construction jobs that will "generate $6.5 billion in new personal income."

The company issued a statement Sunday, saying while it is good to shift to more sustainable energy source, the transition might take decades.

Republican lawmakers, including House Speaker John Boehner, have called on Obama not to delay the decision, which has been kept pending for over four years.

Earlier this month, actress Daryl Hannah along with scientist James Hansen, Robert Kennedy Jr., and several other high-profile personalities were arrested after handcuffing themselves to the gates of the White House to mark their protest.

Some Christians have also opposed the pipeline.

John Elwood, an evangelical and pastor in the Presbyterian Church of America, earlier told The Christian Post the pipeline's path will run through 1,904 rivers and streams – including Nebraska's Ogallala Aquifer – putting the livelihood and drinking water of the poor in harm's way. Referring to Psalm 51:4, he added, "When we do things that affect the poor harmfully, of course we do it to God and when we bless the poor, we do that to God."

A December 2011, Rasmussen Reports survey showed that 53 percent of Americans either strongly favored or somewhat favored the pipeline. Another 2011 Rasmussen poll showed that 75 percent of Americans believe that the U.S. is not doing enough to develop its own traditional gas and oil resources.